1. Field of the Invention
This invention relates to a toner and an image-forming method which are used in electrophotography, electrostatic recording, magnetic recording and toner jetting.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 42-23910 and 43-24748 and so forth are known as methods for electrophotography. In general, fixed images are are obtained by forming an electrostatic latent image on a photosensitive member by various means utilizing a photoconductive material, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer material such as paper as the occasion demands, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor. Also, where the method has the step of transferring the toner image, a step for removing transfer residual toner remaining on the photosensitive member is usually provided, and the above steps are repeated.
Especially in full-color image formation, electrostatic latent images are developed by the use of a magenta toner, a cyan toner, a yellow toner and a black toner, and toner images of the respective colors are superimposed to reproduce multi-color images.
In recent years, the field of utilizing image-forming apparatus making use of such electrophotography is making rapid progress not only in copying machines for merely taking copies of originals, but also in printers used as output means of computers, in personal copying machines for private use and further in plain-paper fax machines, and there are increasing demands in variety. In respect of copying machines, too, they are on the way to higher function by digitization. In particular, on the part of image-forming apparatus, they are being remarkably made smaller in size, higher in speed and more adaptable to color image formation, and further being demanded for higher reliability and higher resolution. For example, those having had a resolution of 200 to 300 dpi (dot per inch) at the beginning are being replaced by those having a resolution of 400 to 1,200 dpi, and further 2,400 dpi.
To meet such demands, image-forming apparatus have come to be designed with simpler components by using members which are highly functional in various respects. As the result, the functionality required for toners has also come to be of a higher order. Thus, under existing circumstances, the improvement in performance of toners must be achieved before any superior image-forming apparatus can be materialized.
For example, in recent years, as transfer assemblies with which toner images on electrostatic latent image bearing members or intermediate transfer members are electrostatically transferred to transfer materials, a case is increasing in which, from the viewpoint of making image-forming apparatus small-sized or preventing ozone from being generated, a contact transfer assembly for performing what is called contact transfer is used. In this transfer assembly, a roller-shaped transfer member to which a voltage is kept applied from the outside is brought into contact with an electrostatic latent image bearing member or intermediate transfer member via the transfer material.
For such a contact transfer assembly, it is preferable for toner particles to be made spherical, from the viewpoint of transfer performance or achievement of high resistance to any mechanical stress coming from the assembly. However, it has turned out that toner particles having a high sphericity have a small specific surface area so as to make a colorant stand poorly dispersed in the interiors of toner particles to greatly affect their transfer performance or matching for the transfer assembly.
Meanwhile, in fixing assemblies for fixing toner images, a heat fixing means on a heat roller system is commonly used which makes use of a pressure roller serving as a rotary heating member and a pressure roller serving as a rotary pressure member (hereinafter called together a fixing roller). Where images are formed at a higher speed, such a fixing roller requires a large quantity of heat energy instantaneously while applying a high pressure. This causes an undesirable situation that the fixing assembly must be made large-sized or its pre-heating time must be set longer. From these viewpoints, it is preferable for toners used in the above image-forming apparatus to exert high sharp-melt performance when heated. Also, such toners not only have a superior fixing performance but also have superior mixing properties when full-color images are formed, and hence have made it possible to broaden the range of color reproduction of the fixed image to be obtained.
However, such toners having good sharp-melt performance commonly have so high affinity for the fixing roller as to tend to cause an offset phenomenon that the toner transfers to the fixing-roller surface at the time of fixing.
This offset phenomenon tends to occur when the fixing temperature is too high or too low. Too high fixing temperature makes the toner have too low viscosity, and hence a toner layer tends to transfer to the fixing-roller surface. Too low fixing temperature makes the toner melt insufficiently, so that the toner can not melt into the transfer material surface to tend to transfer to the fixing-roller surface.
Toners have a temperature range within which favorable fixing can be performed. In actual image formation, however, the temperature of fixing-roller surface may greatly change depending on the environmental temperature when used or the use condition of, e.g., continuous printing on a large number of sheets. Hence, in order to be adaptable to various conditions, it is more preferable for toners to have a broader fixable temperature range.
Even in the case of toners capable of fixing well in a low-temperature region, where the toner layer has come thick on the transfer material, faulty fixing called “image peeling” of about 2 mm in diameter may also occur over several spots on the fixed-image surface, on the side of a low-temperature region. This phenomenon may remarkably occur especially when a color image formed by superimposing a plurality of toners selected from a yellow toner, a magenta toner, a cyan toner and a black toner is formed on the transfer material.
Meanwhile, for the purpose of making the toner not adhere to the fixing-roller surface, for example, a material having good releasability to toners, such as a silicone rubber or a fluorine resin, is used in the surface material of the fixing roller and in addition the fixing-roller surface is covered with a thin film of an offset-preventive liquid in order to prevent the offset phenomenon from occurring and the fixing-roller surface from deteriorating.
The above method is very effective in respect of the prevention of the offset phenomenon. However, difficulties may arise such that (1) since a unit for feeding the offset-preventive liquid is required, the fixing assembly may become complicated to provide a factor of obstruction in designing compact and inexpensive image-forming apparatus; (2) the offset-preventive liquid applied may be permeated into the fixing roller at the time of heating to trigger any peeling at the interface of layers and consequently shorten the lifetime of the fixing roller; (3) since the offset-preventive liquid adheres to the fixed image obtained, a feeling of stickiness may be given and, especially when transparency films utilized in overhead projectors as a means for presentation are used as transfer materials, their transparency may be damaged so that the desired color reproduction cannot be achieved; and (4) the offset-preventive liquid may contaminate the inside of the image-forming apparatus.
In the meantime, transfer materials used in the above image-forming apparatus have also been diversified. For example, as types of paper used as transfer materials, it is the existing condition that not only their basis weights differ but also the materials and contents of raw materials and fillers differ. Some of these transfer materials are composed of materials which are easily desorbed, or tend to adhere to any constituent members of the fixing assembly. Thus, the quality of transfer materials is various. The influence of such transfer materials on fixing assemblies is so great as to make it difficult to make them compact or long-lifetime.
A problem has also arisen such that any contaminants coming from transfer materials and the toner are made into lumps to stick fast to the fixing-roller surface to cause a lowering of the performance of the fixing assembly, or that the matter having stuck may be released to damage the quality of fixed images.
Stated specifically, regenerated paper making use of regenerated pulp obtained by deinking paper once used has come to be in wide use from the viewpoint of environmental conservation. However, the regenerated paper often contains various adultalants or impurities. For example, Japanese Patent Application Laid-Open Nos. 3-28789, 4-65596, 4-147152, 5-100465 and 6-35221 disclose techniques relating to regenerated paper. As disclosed therein, the content and constitution of the adultalants or impurities in the regenerated paper must be specified in order to make it usable in the above image-forming apparatus.
At present, in regenerated paper used in usual office work and so forth, the mixing proportion of regenerated pulp made from newspaper waste paper and so forth is more than 70%. It is predicted that its mixing proportion increases more and more in future, and it is apprehensive that this may cause the above problems. In addition, where a cleaning member for removing toner and so forth having adhered to the surface of the heating roller or a separation member for preventing transfer materials from winding around the roller is provided, it has been ascertained that the surface of the fixing roller may be scratched or abraded or the function of the cleaning member or separation member may greatly lower, because of mechanical-paper type pulp fibers contained in paper dust desorped from, in particular, regenerated paper made from mechanical waste paper such as newspaper and magazine waste paper. Such a phenomenon tends to cause a serious question when using a fixing assembly in which the offset-preventive liquid is applied on the fixing roller in a small quantity or a fixing assembly in which any offset-preventive liquid is not applied.
As stated above, the application of the offset-preventive liquid on the fixing-roller surface of the fixing assembly is very useful, but on the other hand has various problems.
Taking account of the demand made recently on image-forming apparatus, such as miniaturization and weight reduction, and the quality of fixed images, it is preferable to remove even such an auxiliary unit for applying the offset-preventive liquid.
Under such circumstances, it is essential to make technical development relating to the heat-and-pressure fixing of toners, and some measures therefor have been proposed.
Conventionally, from the idea that the offset-preventive liquid should be fed from the interiors of toner particles at the time of heating, without use of any unit for feeding the offset-preventive liquid, methods of incorporating a wax component such as low-molecular-weight polyethylene or polypropylene in toner particles have been proposed in a large number. However, such a wax component must be added to the interiors of toner particles in a large quantity in order to bring out a sufficient effect. In that case, filming on the photosensitive member or contamination of carrier surface or developer-carrying member surface may occur to cause new problems such as image deterioration. Also, where the wax component is added in a small quantity, it turns necessary to juxtapose a unit for feeding the offset-preventive liquid in some quantity or an auxiliary cleaning member such as a wind-up type cleaning web or a cleaning pad. When transparency films are used as transfer materials especially in forming full-color images, the fixed images may have poor transparency or haze because of high crystallization of the wax component or difference in refractive index from binder resins. This problem has remained unsolved.
Japanese Patent Publication Nos. 52-3304 and 52-3305, Japanese Patent Application Laid-Open Nos. 57-52574, 60-217366, 60-252360, 60-252361, 61-94062, 61-138259, 61-273554, 62-14166, 1-109359, 2-79860 and 3-50559 also disclose techniques of incorporating wax in toner particles. If, however, the wax is merely incorporated in toner particles, it is difficult to highly improve various properties required for toners, and the matching for image-forming apparatus making use of the heat-and-pressure fixing system can not be satisfactory.
Meanwhile, there is also an attempt to improve fixing performance by improving a binder included in the toner. For example, a toner is proposed in which a rosin and a rosin derivative which have originally been used in toners as pigment-dispersing agents are added for the purpose of improving fixing performance.
Japanese Patent Application Laid-Open No. 2-173760 and Japanese Patent Publication No. 7-82254 disclose that a polymer having a rosin compound as a condensation component is used as a binder resin. The rosin and the rosin derivative present in toner particles can improve fixing performance to a certain extent. However, in the case of using the fixing assembly in which the offset-preventive liquid is applied in a small quantity or the fixing assembly in which any offset-preventive liquid is not applied, there is room for improvement.
Japanese Patent Applications Laid-Open Nos. 5-333595 and 2000-109189 also disclose that the quantity of a cross-linking agent to be added is controlled to improve fixing performance, but do not mention any rosin compound and its combination with any rosin compound derivative.
Now, in the present technical field, it is known to use various pigments or dyes as colorants for the purpose of improving the color reproducibility of color toner images.
Magenta toners are important in order to use them together with yellow toners to reproduce red color, to which humans have a high visual sensitivity, and required to have good developing performance when, e.g., the flesh color of portraits having a complicated color tone is reproduced. Also, when used together with cyan toners, the second color reproduction of blue color used highly frequently as business color must be achieved.
Conventionally, as the magenta toner, it is known to use any of quinacridone colorants, thioindigo colorants, xanthene colorants, monoazo colorants, perylene colorants and diketopyrrolopyrole colorants alone or in the form of a mixture. For example, Japanese Patent Publication No. 49-46951 discloses a toner making use of a 2,9-dimethylquinacridone pigment; Japanese Patent Application Laid-Open No. 55-26574, a thioindigo pigment; Japanese Patent Application Laid-Open No. 59-57256, a xanthene pigment; Japanese Patent Application Laid-Open No. 11-272014, a monoazo pigment; Japanese Patent Application Laid-Open No. 2-210459, a diketopyrrolopyrole pigment; and Japanese Patent Application Laid-Open No. 55-42383, an anthraquinone pigment.
However, it is not the case that these colorants fulfill all the conditions required for magenta toners. There has been room for improvement in respect of any of toners' color tone, light-fastness and charging performance and also their matching for image-forming apparatus.
It has also been necessary to take into account such a problem that any good negative chargeability can not be achieved because of charge characteristics of colorants used and hence the toner may scatter from the developing assembly to adhere to, and contaminate, the inside of the image-forming apparatus.
Japanese Patent Application Laid-Open No. 1-22477 discloses a method in which a quinacridone organic pigment and a xanthene dye are used in combination and Japanese Patent Application Laid-Open No. 2-13968 discloses a method in which a quinacridone colorant and a methine colorant are used in combination, both to obtain a toner of sharp magenta color and improve the charging performance and light-fastness of the toner and further to prevent the fixing roller such as a silicone rubber roller from being dyed. Japanese Patent Application Laid-Open No. 62-291669 (corresponding to U.S. Pat. No. 4,777,105) further discloses a toner making use of a quinacridone pigment which is in a state of mixed crystals.
Japanese Patent Application Laid-Open No. 11-52625 also discloses a method in which a red pigment classified as a sort of C.I. Pigment Red 48 and a red pigment such as quinacridone pigment having a b* value of −5 or less in the L*a*b* color system are used in combination in a mixing ratio of from 2 to 30% by weight to obtain a toner of good magenta color, improve charging performance and light-fastness of the toner and further improve its heat resistance to the fixing roller.
Meanwhile, many agents are known as colorants for yellow toners. For example, Japanese Patent Application Laid-Open No. 2-207273 discloses dyes such as C.I. Solvent Yellow 112; Japanese Patent Application Laid-Open No. 2-207274, C.I. Solvent Yellow 160; and Japanese Patent Application Laid-Open No. 8-36275, C.I. Solvent Yellow 162. Japanese Patent Application Laid-Open No. 50-62442 further discloses a benzidine yellow pigment; Japanese Patent Application Laid-Open No. 2-87160, a monoazo yellow toner; and Japanese Patent Application Laid-Open No. 2-208662, pigments such as C.I. Pigment Yellow 120, 151, 154 and 156.
The above yellow dyes commonly have superior transparency, but have inferior light-fastness, and may cause not only a problem on storage stability of images but also a problem on the combination with image-forming apparatus.
On the other hand, the above yellow pigments commonly have superior light-fastness as compared with the yellow dyes, but have room for improvement in respect of the state of dispersion in toner particles, and hence have a problem that image fog due to poor charging performance may occur.
Yellow pigments having superior light-fastness or heat resistance may also commonly make toners have so extremely low transparency that OHT (overhead projector transparency film) projected images tend to have a dullness.
Japanese Patent Publication No. 2-37949 discloses a disazo compound having superior light-fastness and its production method. This belongs to a group of compounds typified by C.I. Pigment Yellow 180, and has superior light-fastness and heat resistance.
As other examples in which C.I. Pigment Yellow 180 is used, they may include those disclosed in Japanese Patent Application Laid-Open Nos. 6-230607, 6-266163 and 8-262799. The toners making use of the above pigments, however, can by no means be said to have good transparency, and have not been designed also taking account of fixing performance. Thus, they must further be improved as yellow toners for full-color image formation.
Japanese Patent Application Laid-Open No. 8-209017 discloses a toner for electrophotography in which, in order to solve the above problems, a yellow pigment obtained by making a yellow pigment into fine particles to improve its specific surface area is used to improve transparency and coloring powder. However, making fine the pigment classified as C.I. Pigment Yellow 180 may inevitably result in a great lowering of negative chargeability of the pigment itself. Toners making use of such a pigment have caused a new problem of lack of charge quantity, in particular, lack of charge quantity in an environment of high temperature and high humidity.
The above colorant is also so strongly self-agglomerative that it may disperse with difficulty in a good state in the binder resin included in the toner. Hence, there has been a tendency of showing inferior OHT transparency.
In addition, as a result of studies made by the present inventors on fixing performance of toners containing the colorants mentioned above, it has been found that a tendency of narrowing the fixable-temperature range is seen or image peeling tends to occur. In respect of the toners containing the colorants listed above, their influence on fixing performance is little taken into account. In particular, nothing is taken into account at all on the case in which regenerated paper making use of regenerated pulp in a mixing percentage of more than 70% is used as the transfer material, on the occasion of color image formation where a plurality of toner layers formed on the transfer material must be fixed at a time, on the case in which the fixing assembly in which the offset-preventive liquid is applied on the fixing roller in a small quantity or the fixing assembly in which any offset-preventive liquid is not applied is used, or on how to adapt them to the contact development system.
As having been discussed above, there has not yet been any sufficient measure to generally deal with the system designing of image-forming apparatus making use of the heat-and-pressure fixing system, taking even the developing system into account including colorants used in toners.
In particular, under the eristing circumstances, the above magenta toner and yellow toner, inclusive of their secondary colors, have not yet reached any sufficient level in respect of the transparency and so forth.